RADIO IN OBSERVING AURORAS

Radio & Auroras

The interaction between Earth's magnetic field and
Solar particles is a complex and mysterious field of science. The storm
events involve high electric currents in the ionosphere and vast amounts
electric power affecting to great many things. One of the ways to observe
what is happening up there, is to detect the effects of these phenomena
to non ionizing long wave electromagnetic radiation - radio waves. Earth's
magnetosphere it self is a source of electromagnetic RF energy at LF- and
VLF frequencies, but since these emissions are not easy to detect from
other emissions specially in rural areas, a good alternative is to monitor
radio signals sent by man. Ionosphere on high latitudes is affected during
geomagnetic disturbances, which can be observed on radio signals that pass
through, or become reflected from the E layer of the ionosphere. Visible
Auroras, which are observed at higher altitudes and require less less energy
to appear, as well as Radio Auroras, requiring extremely dense ionization
around 100 km to occur, are just some of the phenomena related to geomagnetic
disturbances. The origin of these disturbances are the enhancements of
density, energy, quality and speed in Solar particle emissions, including
the polarity of the interplanetary magnetic field and coupling of the particles
to Earth's magnetosphere.

Types of radio Auroras

On HF (3...30 MHz), and more notably on LF and VLF (30...300 kHz)
bands the Aurora causes the field strength (F/S) of distant radio
signals
to fluctuate rapidly, while during quiet days no such transients are
observed. Other common Solar particle related phenomena are polar cap absorption
(PCA) and short-wave fadeouts (SWF).

On VHF frequencies (30...300 MHz) the most notable type of Radio
Aurora is the
Auroral back scatter of radio signals from pillars
and arcs of collisional ionization in the E layer concentrated by ionic-acoustic
waves at about 100 km heights. These reflecting irreqularites in the ionosphere
are only of meter class in size. This may sound odd as the sky might simultainously
be filled up with curtains of visual auroras with peak heights up to several
hundred kilometers.

The propagation geometry is strictly (magnetic) field aligned
(-10 dB/°). Such signals are detected up to UHF, but are more
obvious on lower VHF frequencies. Exotic Auroral E propagation,
occurring
perhaps by the 10 keV electrons entering the cleft and becoming deposited
at the midnight precipitation zone (Thule type Auroral E), sometimes
occurs a few hours after night Radio Aurora, but with signal strengths
that are sufficient only to be detected reliably on VHF low band
(approximately
30...70 MHz).

Auroral back scatter signals are caused more directly from the Solar
particle emissions captured by Earth's magnetic field and are being received
often around 1600 local time. These afternoon Radio Auroras have
plenty of Doppler-spread dispersing the signal's spectrum. This
effect is caused by the vast mass of individual gyrating electrons
reflecting the radio wave. There also can be observed Doppler shift or in
fact several
of them caused by the overall motion of the ionized reflecting areas in
relation to the location of transmitter and receiver due to the electric
field. During disturbed conditions two peak spectra can be observed from
differences caused by diverse electric field conditions at differenct
altitudes. The typical Doppler phase velocities range from -1000 to +1000 m/s.

A minima called the Harang discontinuity occurs around 2100 local
time.

Another maxima is at midnight hours, with less Doppler hiss on the signals
and slightly less strength. The particles are more of deposits from the
magnetotail, rather than being related directly to Solar emissions. High
local geomagnetic indices (K=9) and Radio Aurora are rarely observed during
the late morning hours. Basically the same Radio Auroras are observed also
on the HF part (3...30 MHz) of radio spectrum, but since other ionospheric
propagation modes (F, Es) are often dominant there and noise level is higher,
the interpretation of propagation mode(s) becomes difficult. A suitable
set-up on VHF does not have any other than ionospheric propagation. Of
these, the F layer reflections do not occur above 60 MHz and Es appears
sporadically in the summer months and reducing as frequency increases,
leaving in practice only Meteor Scatter and Radio Aurora as primary propagation
modes.

You may study more on the behavior and physics of the Auroras from
Space Physics Textbook by the University of Oulu.

Radio observations in the Aurora section

While amateurs have observed Radio Auroras for decades, the section has
been active before only in visual observing. The first radio observations
were sent to the Ursa Aurora Section by Mr. Reino Multanen in 1993. He
received the U.K. LF (Low Frequency) time standard station MSF on 60 kHz,
monitoring F/S (Field Strength) variations with a pen recorder.

Ilkka Yrjölä has observed Radio Aurora (24 h/day) on 87 MHz
since August 1993 (on various frequencies around 89 MHz since April 1998)
and on 144 MHz since January 1995, using automated set-up of PC, two (since
1998 three) receivers and squelch detection. This effort is a by-product
of Global-MS-Net searching for meteor outbursts and monitoring meteor rates
by receiving Meteor Scatter signals, which are sometimes disrupted
by Radio Aurora. Auroral data from this effort has been regularly included
in the Aurora Sections reports in the form of bar charts.

SK4MPI Radio Beacon

VHF
back scatter from Aurora.

Receiving the Swedish Radio Aurora beacon SK4MPI on 144.412 MHz
is one of the most sensitive and reliable ways for an amateur to detect
Radio Aurora in Scandinavia. This beacon can usually be heard when the
K index rises to 4 or higher. Observations on 50 MHz with large high gain
antennas have revealed weak Auroral signals being received at high latitudes
whenever the geomagnetic field is even slightly unsettled (K 1 or 2)

SK4MPI beacon located in Borlänge, Sweden, is dedicated to aid
detection and study of Radio Aurora conditions on VHF in northern Europe.
The beacon was originally initiated in the early 1970's by Max-Planck-Institute
fur Aeronomie for scientific Aurora research and is now maintained by Amateur
Radio operator SM4HFI. SK4MPI is the most powerful continuously transmitting
Amateur Radio beacon in Europe on
the 144 MHz band.

It has ERP of 1.5 kW to two broad beams towards N/W and
N/E. The benefit of using this station to detect Aurora, is that there
are no other transmissions on the same frequency.

The beacon is not received in Kuusankoski, Finland, if there is no Aurora,
except for some short and random meteor reflections. As Auroral conditions
begin, back scattered signal comes up from noise. With moderate receiving
antennas the signal may peak up to
20 dB above noise level.
The beacons keying cycle consists of a 1 minute long message with A1A Morse
coded call sing & locator and the rest 45 seconds is carrier signal
(A0).

Green curve indicates strength (large antenna) and bar the detection
(smaller antenna) exceeding expressed threshold of back scatter signal
from Sweden. Red bar shows detection of signals from Germany on 87 MHz
and cyan the detection of Dutch TV carrier on 62.2 MHz. Notice the later
appearance of long distance signals from west compared to back scatter
from north.

To receive the signal, a narrow band receiver for 144.412 MHz is needed
along with a Yagi antenna with 4, or more elements. A 12 element Yagi
and a narrow band FM receiver with -125 dBm threshold, has been used
to good effect on this project. A location with low RF noise and free takeoff
towards north is of course necessity. Converting F/S information from the
receiver with an A/D converter and feeding it to a computer that
logs
the data, is one way to gather information on Radio Aurora. Sampling
can be relatively slow and F/S signal can be heavily integrated.

Examples of Aurora signals

Audio spectrogram of SK4MPI, a 144.412 MHz Amateur Radio beacon for
aurora taken 14.11.1997 by OH5IY. The pure narrow carrier is smeared by
Doppler-spread effect in to noise several hundreds of herzes wide.

Spectrum of two TV carriers on 49.76 MHz. The lower frequency means
there is less Doppler-spread. The slow wobbling of the carrier originates
from the transmitters. The typical fast flutter is from Aurora's Doppler-shift.